1. Field of the Invention
This invention relates a method of controlling a two-degree-of-freedom control system, and more particularly to a method of controlling a two-degree-of-freedom control system that controls a VCM (voice coil motor) of a magnetic disk apparatus.
2. Description of the Related Art
A two-degree-of-freedom control system is generally carried out in order to constitute a tracking system in a mechanical system and the like. For example, as shown in FIG. 1, the two-degree-of-freedom control system 107 includes an actuator 100 as a controlled object, a feedback controller 101, a feedforward controller 102 for generating a control signal 105 given to the actuator 100, and a feedforward controller 103 for generating a reference signal 106 by a step-formed target value 108 given to a feedback loop. Recently, because the two-degree-of-freedom control system is usually constituted by a digital control system equipped with a micro-computer, the feedforward controller 102, the feedforward controller 103, the feedback controller 101 etc. can mathematically be parts of the micro-computer and the outputs thereof are calculated by the micro-computer. The control command generated from the outputs are given to the driver of the actuator 100 through a D/A converter.
In the two-degree-of-freedom control system 107 as shown in FIG. 1, the actuator 100 is driven by the control signal 105 from the feedforward controller 102, and the difference between a measured position signal from the actuator 100 and the reference signal 106 from the feedforward controller 103 is calculated and applied to the feedback controller 101. The outputs from the feedback controller 101 and the feedforward controller 102 are then entered to a zero order holder. It should be understood from the above explanation that the frequency components of the control signal 105 from the feedforward controller 102 should not excite (or generate) a resonance mode such as a high order resonance mode. Further, the control signal including less frequency components in a condition of the high order resonance mode is necessary for the actuator 100 from the feedforward controller 102.
In order to realize the tracking system with high speed in such a system, the frequency characteristic of the actuator 100 as the controlled object becomes important. If an actuator with better characteristics and a frequency band much higher than the control band required for the high order resonance mode exists, it is not difficult to design a tracking system with high speed by using the digital control system. However, in order to realize the two-degree-of-freedom control system with high performance, the control band should be set as high as possible.
It is unknown, without changing mechanical design, to set the high order resonance mode of the actuator as the controlled object to a frequency band higher than the control band, and the high order resonance mode of the actuator tends to be in the frequency band close to the required control band. If the tracking control with high speed is realized, a control system designed by taking account of the influence of the high order resonance mode of the actuator is required. In the case where the sampling frequency of the digital control system is high enough and the frequency of the high order resonance mode is lower than Nyquist frequency, it is possible to design the control system, in the designing of the feedback controller and the feedforward controller, by taking the influence of the high order resonance mode using H .infin. control theory into consideration.
However, if such a mechanical system in which the sampling frequency of the digital control system cannot be made high enough due to the restraint of the system and the frequency of the high order resonance mode is higher than the Nyquist frequency, it is difficult to take the high order resonance mode of the actuator into consideration when designing the control system. Therefore, it is difficult to realize the tracking control.
As described above, in the conventional method of controlling the two-degree-of-freedom control system, the high order resonance mode of the actuator as the controlled object exists in a frequency band that is higher than the Nyquist frequency. Therefore, if the high order resonance mode tends to be close to the control band, it is difficult to realize the tracking control with high speed, and in the worst case, vibration occurs between sampling points.